This invention relates to methods and apparatus for routing wavelengths in a photonic network.
In any optical or photonic network it may be expected that signal degradation occurs as signals are passed through nodes in the network. This is because in contrast to a conventional (non-optical) network, signals may be routed using entirely optical means and therefore do not necessarily undergo regeneration at each network node. Generally speaking, the degradation is correlated to the number of nodes which the signal has passed through. For example, in a photonic network a wavelength typically may be permitted to pass through a predetermined maximum number of amplifiers before the noise added at each amplifier becomes unacceptably high. Therefore in order to permit the use of long paths with many nodes, it is necessary to periodically regenerate the signal to maintain an adequate signal quality (specified for example in terms of an acceptable bit error rate [BER] or noise floor).
In the case of a wavelength division multiplexed (WDM) arrangement, regeneration typically takes the form of an optical-electrical-optical (OEO) conversion. Thus, in routing a path through a photonic network carrying such multiplexed wavelengths, it is necessary to provision regenerative nodes at suitable intervals along the path. However, it will be noted that a node (such as an optical cross connect [OXC]) carries an additional cost over that of a simpler, optically transparent photonic cross connect (PXC) or an amplifier. Thus It is desirable to minimise the use of regenerative nodes in any cone path in order to reduce overall network costs.
Thus in principle, it is desirable to cause each path through the network to traverse as many amplifiers as possible before being regenerated (in order to reduce the number of regenerative nodes required) but to ensure that no path is allowed to traverse mote than a predetermined maximum number of amplifiers (that number being selected to ensure that the signal is not excessively degraded before regeneration).
In practice, selecting such paths particularly in a mesh network with many routing choices, is very complex.
Algorithms have been developed which are suitable for use in a centralised planning arrangement where network configuration is controlled centrally and is only periodically altered. However, it is desirable to provide an agile dynamic deployment of wavelengths across a photonic network and prior art algorithms are too cumbersome for such uses.
In accordance with a first aspect, the invention may provide a network configuration tool for a photonic network comprising a route generator operable to generate a list of possible paths through the network between a first and second node in the network, an non-regenerative node counter operable to determine for each of the possible paths whether the respective path traverses a number of non-regenerative nodes which is greater than a predetermined maximum number of non-regenerative nodes, a non-regenerative path selector operable to select a path from the set of paths which have been determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, and to output the selected path, a path rejecter operable to reject any paths which are determined to traverse a number of non-regenerative nodes which is greater than the said maximum non-regenerative node value and which do not traverse regenerative nodes, a Path shortener operable to generate for each possible path, a set of shortened paths, each shortened path passing between the said first node and a regenerative node in the respective possible path, the non-regenerative node counter being further operable to determine for each shortened path whether the shortened path traverses a number of non-regenerative nodes which is greater than the predetermined maximum number of non-regenerative nodes, and a regenerative path selector operable to select a regenerated path by choosing a possible path which corresponds to a shortened path determined not to traverse a number of non-regenerative nodes which is greater than the predetermined maximum number of non-regenerative nodes, the regenerated path being provisioned to include a regeneration at the node corresponding to the end node of the shortened path, and the regenerative path selector being further operable to output the selected regenerative path.
The invention may in a second aspect, provide a network manager for a photonic network comprising an non-regenerative node counter operable to determine for each of a plurality of possible paths through the network between a first and second node in the network, whether the respective path traverses a number of non-regenerative nodes which is greater than a predetermined maximum number of non-regenerative nodes, a non-regenerative path selector operable to select a path from set of paths which have been determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, and to output the selected path, a path rejecter operable to reject any paths which are determined to traverse a number of non-regenerative nodes which is greater than the said maximum non-regenerative node value and which do not traverse regenerative nodes, a path shortener operable to generate for each possible path, a set of shortened paths, each shortened path passing between the said first node and a regenerative node in the respective possible path, the non-regenerative node counter being further operable to determine for each shortened path whether the shortened path traverses a number of non-regenerative nodes which is greater than the predetermined maximum number of non-regenerative nodes, and a regenerative path selector operable to select a regenerated path by choosing a possible path which corresponds to a shortened path determined not to traverse a number of non-regenerative nodes which is greater than the predetermined maximum number of non-regenerative nodes, the regenerated path being provisioned to include a regeneration at the node corresponding to the end node of the respective shortened path, and the regenerative path selector being further operable to output the selected regenerative path.
In another aspect, the invention provides a method of selecting a route through a photonic network comprising selecting nodes in the route based on the signal degradation between optical regenerators in the route. The signal degradation may be measured by counting non-regenerative nodes. Alternatively or additionally, measurements of the signal may be made such as BER or noise, and the measurement may be compared with a threshold beyond which provisioning of regeneration in the route is considered necessary. The measurements may be made, for example, using the techniques disclosed in Nortel Networks co-pending U.S. patent application xe2x80x9cOptical Networks with Signal Regenerationxe2x80x9d, (Nortel reference 12947D) which was filed on Dec. 6, 2000.
In a further aspect of the invention there is provided a method of selecting a wavelength route between a first and a second node in a photonic network comprising the steps of determining for each of a plurality of possible paths between the first and second nodes, whether the respective path traverses a number of non-regenerative nodes which is greater than a predetermined maximum number of non-regenerative nodes, for the set of possible paths determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, selecting a non-regenerated path from the set, if all possible paths are determined to traverse a number of non-regenerative nodes which is greater than the said maximum non-regenerative node value, rejecting any paths which do not traverse regenerative nodes, generating for each possible path, a set of shortened paths, each shortened path passing between the said first node and a regenerative node in the respective possible path and determining for each shortened path whether the shortened path traverses a number of non-regenerative nodes which is greater then the predetermined maximum number of non-regenerative nodes, for any shortened paths determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, selecting a regenerative path by choosing a possible path which corresponds to such a shortened path and which is provisioned to induce a regeneration at the node corresponding to the end node of the respective shortened path.
This method quickly rejects paths which have no hope of being suitable either because there are too many non-regenerative nodes (such as amplifiers) and no possibility of regeneration or because the regenerative nodes (such as OXCs) are too widely spaced apart in the path.
This basic algorithm will select a path which has sufficient regenerations or will indicate that no such paths are possible. However, it is desirable to attempt to reduce the number of regenerations by ensuring that the path traverses as many amplifiers as possible before being regenerated. This could be considered to be an optimum path.
Algorithms are known (for example, the modified Dijkstra (R Bhandari, Proceedings, IEEE info com 1498-1508, 1994) which are able to calculate a xe2x80x9cshortestxe2x80x9d path between two points and a network. The algorithm operates using parameters representing link xe2x80x9ccostxe2x80x9d between each node. The cost or xe2x80x9clengthxe2x80x9d may be weighted to favour particular links. Initially, for example the xe2x80x9ccostxe2x80x9d may be directly related to the physical length between the nodes. However it may be operationally expedient to adjust the length or costs of the links in order to control provisioning of paths across those nodes forming the ends of those links.
For situations in which the algorithm determines that no regeneration is necessary, the path may be provisioned based simply on link cost in the normal way.
However, for paths for which regeneration is required, the algorithm ideally operates to ensure that paths having few non-regenerative nodes between the regenerative nodes are less favoured than paths which have more non-regenerative nodes between the regenerative nodes. This may be achieved in accordance with the invention by reversing the sign of the link cost for each link in each shortened path which has been determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes and using the reverse sign value selecting the shortest path between the first and second node. The modified Dijkstra algorithm for example can operate with negative link cost value.
In this way, a path having a long xe2x80x9cshortened pathxe2x80x9d i.e. a long distance between regenerative nodes, will appear to xe2x80x9ccostxe2x80x9d less than that having a small number of non-regenerative nodes. This is because, as explained below, the long shortened path once the sign of its link costs is reversed, has a very low value relative to a shorter xe2x80x9cshortened pathxe2x80x9d.
In another aspect, the invention provides a node in a photonic network forming part of a wavelength routing path between a first and second node in the network, the node having been selected by determining for each of a plurality of possible paths between the first and second nodes, whether the respective path traverses a number of non-regenerative nodes which is greater than a predetermined maximum number of non-regenerative nodes, for the set of possible paths determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, selecting the node as part of a non-regenerated path from the set if all possible paths are determined to traverse a number of non-regenerative which is greater than the said maximum non-regenerative node value, rejecting any paths which do not traverse regenerative nodes, generating for each possible path, a set of shortened paths, each shortened path passing between the said first node and a regenerative node in the respective possible path and determining for each shortened path whether the shortened path traverses a number of non-regenerative nodes which is greater than the predetermined maximum number of non-regenerative nodes, for any shortened paths determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, selecting the nod as part of a regenerated path by choosing a possible path which corresponds to such a shortened path and which is provisioned to include a regeneration at the node corresponding to the end node of the respective shortened path.
In other aspects, the invention provides a computer program which when executed on hardware associated with a photonic network, causes the hardware to determine for each of a plurality of possible paths between first and second nodes in network, whether the respective path traverses a number of non-regenerative nodes which is greater than a predetermined maximum number of non-regenerative nodes, for the set of possible paths determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, select a non-regenerated path from the set, if all possible paths are determined to traverse a number of non-regenerative nodes which is greater than the said maximum non-regenerative node value, reject any paths which do not traverse regenerative nodes, generating for each possible path, a set of shortened paths, each shortened path passing between the said first node and a regenerative node in the respective possible path and determining for each shortened path whether the shortened path traverses a number of non-regenerative nodes which is greater than the predetermined maximum number of non-regenerative nodes, for any shortened paths determine not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, select a regenerated path by choosing a possible path which corresponds to such a shortened path and which is provisioned to include a regeneration at the node corresponding to the end node of the respective shortened path and/or a switch communications network a switched communications network operable to determine for each of a plurality of possible paths between first and second nodes in the network, whether the respective path traverses a number of non-regenerative nodes which is greater than a predetermined maximum number of non-regenerative nodes, for the set of possible paths determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, select a non-regenerated path from the set, if all possible paths are determined to traverse a number of non-regenerative nodes which is greater than the said maximum non-regenerative node value, reject any paths which do not traverse regenerative nodes, generating for each possible path, a set of shortened paths, each shortened path passing between the said first node and a regenerative node in the respective possible path and determining for each shortened path whether the shortened path traverses a number of non-regenerative nodes which is greater than the predetermined maximum number of non-regenerative nodes, for any shortened paths determined not to traverse a number of non-regenerative nodes which is greater than the said maximum number of non-regenerative nodes, select a regenerated path by choosing a possible path which corresponds to such a shortened path and which is provisioned to include a regeneration at the node corresponding to the end node of the respective shortened path.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.
In the invention described herein, regenerative nodes are taken to mean nodes which have the capability of regenerating a signal passing through them but which may not be used for that purpose. For example a node may be a combined photonic cross-connect and optical cross-connect. By provisioning a route through the photonic cross-connect which is optically transparent and does not involve an OEO conversion, costs are reduced. However, using the algorithm it may be determined that regeneration is required and in this case the same node may be used but the route may be provisioned through the optical cross-connect function of the node. It will also be appreciated that xe2x80x9coptical regenerationxe2x80x9d is possible in which regeneration occurs entirely in the optical domain. This invention is equally applicable to this type of regeneration since it is still desirable to reduce the number of such regenerations which are required.
A xe2x80x9cnon-regenerativexe2x80x9d node is taken to mean any network node (e.g. an amplifier) which has no regenerative capability.